In, as a preposition of place, is usually used to talk about the position of someone or something inside large places such as countries, continents, big cities etc. For example:- She grew up in New Zealand. At, as a preposition of place, is usually used to talk about the position of someone or something inside small and unimportant places such as villages, small towns etc. For example:- I'lll meet you at the pub. At is also commonly used with proper names such as buildings or organizations. For example:- I first met her at Harrods. She works at the bank. In, as a preposition of time, is usually used when talking about parts of the day. For example:- I'll see you in the morning, in the evening, in the afternoon. At, as a preposition of time, is usually used with clock time. For example:- My train leaves at 6.30 am. source:http://www.learnenglish.de/mistakes/atvsin.htm

"in" means that you are physically inside the place. "at" means you are nearby or around. I am in the park. - this means that you are actually inside the area. I am at the park - this means that you are around or inside the area. "I am in class" vs "I am at class". Same logic as above.

I will just list examples so you can learn that way: In brazil, the people speak Portuguese. I was born in London. I grew up in Singapore. She was in France over the Christmas holiday. What country is the city of Krakow in? Examples of how to use "at" as you preposition when talking about place: My parent's store is at the corner of Columbus and Broadway. I will be at the library all afternoon. She met me at my house before the hockey game. I went to high school at the age of 14. Let's meet at 4 p.m. They worked at the hospital for 7 years. I was at the Miami Airport when my cousin called to say that he was in a car accident. Did she used to work at the Italian restaurant on route 37? I hope this helps you :)